1. Field of the Invention
This application relates generally to computer animation and specifically to computer animated faces.
2. Description of the Related Art
Computer animations typically comprise a succession of frames. Each frame comprises a collection of “bones” and “vertices.” “Bones” are data structures associated with groups of “vertices” such that as the bones move with each frame, so do the associated vertices. In this manner, the vertices of a mesh may be drawn and displaced on-screen based on bone movements, giving the impression of a dynamically moving mesh without having to individually dictate the motion of each vertex. The vertex mesh may be arranged in the shape of a face, as is the basis for computer character animation. For example, a “jaw bone” may be used to move vertices along the mouth to simulate speech.
Imbuing computer characters with the appearance of real human emotions has traditionally involved capturing the performance of an actor using a marker-based or similar system (also referred to as “motion-capture”). The 3D data derived from the marker capture is then mapped to a model mesh on a computer. Marker-based systems capture the actor's performance by placing small illuminated markers on the actor at points of interest. Precisely placed digital cameras are then used to record the markers' movements from different angles while the markers are illuminated during the performance. Motion capture systems thus capture the movement of a real object and translate that movement to a computer-generated representation. Such captures have been used in the motion-picture and the game industries to create source data used to lend authenticity to synthetic and otherwise noticeably artificial creations.
Live-actor motion-capture may be mapped to synthetic facial models, or to models derived from other actors through “retargeting” methods. Animation retargeting is a process by which animation data from one collection of bones (known as a skeleton) is applied to another collection of bones. That is, the captured points of interest of a real-world performance will be mapped to the equivalent bones and vertices on a separately created synthetic model. This effectively causes the synthetic model to execute the actor's performance, albeit with lower fidelity to the original. Because the synthetic model to which the live performance is to be retargeted often comprises relatively fewer vertices and bones, only a handful of markers are necessary.
While adequate for most motion-picture and entertainment needs, certain applications require high fidelity representations of an actor's performance. One such application is the depiction of microexpressions, minute involuntary physical changes on an individual's face, typically as a consequence of a suppressed emotion. Microexpressions typically last a few tenths of a second, such as less than half a second. For example, a microexpression can be less than 0.25 seconds, less than 0.2 second, or less than 0.1 seconds.
The psychologist Paul Ekman helped categorize these involuntary responses with the creation of a procedure to analyze human facial expressions, known as the facial action coding system (FACS). FACS linked most of the known emotions to specific muscle movements in the face, known as Action Units, which work together to achieve a particular expression. FACS identifies 72 such units and explains how they are triggered and what combinations result in specific emotions. FACS conveniently serves as an “alphabet” of emotional expression components and has been used both by psychologists and computer scientists to structure an otherwise amorphous set of emotions.
Because they indicate a suppressed emotion, microexpressions may be used to detect the statement of falsehoods, or ulterior motives. Some individuals are trained to recognize such microexpressions as part of interrogation or interview processes. Typically, these individuals must undergo extensive training with a live trainer who is already familiar with the techniques.